let calc (func:double -> double) (l,r, step : double)  threadNumber:int =
 let n = (r-l)/double threadNumber
 let res =  ref 0.0
 let threadArray = Array.init threadNumber ( fun x ->
  new Thread(ThreadStart(fun _ ->
    let threadRes = squareOnRange (func) (l+double x*n) (l+(double x+1.0)*n) step
    Monitor.Enter(res)
    res := !res + threadRes
    Monitor.Enter(res)
   )
  ))
 for x in threadArray do
  x.Start()
 for x in threadArray do
  x.Join()
 res.Value